1. The Field of the Invention
This invention relates to lasers that use dispenser cathodes that are heated in order to function as a source of electrons with which to induce laser activity. More particularly, the present invention relates to methods and apparatus for stabilizing the mirror alignment in such lasers despite the temperature-induced dimensional changes caused in the structure of the laser adjacent to the dispensing cathode by the heat generated therein.
2. The Prior Art
The use of laser technology in all areas of human activity is on the increase. This has resulted in a demand for equipment which can produce laser energy at a reasonable cost and on a reliable basis. In all lasers, electromagnetic phenomena are used to excite atoms in an active region of the laser. The relaxation of that excitation then produces light energy at a single predictable wavelength. This light energy is then reflected back and forth through the active region in order to give a single direction to subsequently produced light energy. The result is an intense beam of highly coherent light energy which may be applied to any number of scientific, industrial, or medical purposes.
Some lasers employ dispenser cathodes, which are heated and then emit free electrons. Typically, dispenser cathodes are made of tungsten impregnated or matrix-mixed with barium. When an electrical current is passed through such an electrode, the resistance in the tungsten causes heat in the manner of an incandescent light bulb filament. Once heated, the barium impregnations in the tungsten give off free electrons. These are then directed to flow through the active region of the laser to an anode placed on the opposite side of the active region from the cathode. Passing through the active region of the laser, this stream of electrons excites the atoms of the gas therein, which in turn results in the generation of light energy.
A typical known laser of this type is shown schematically in FIG. 1, where a laser tube A can be seen to include an active region B formed longitudinally at the center of a ceramic core C. Aligned with and at opposite ends of active region B are first and second mirrors D, E, respectively. A gas in active region B produces laser energy which is reflected between first and second mirrors D, E, respectively, to develop in active region B resonant light emissions of intense and coherent qualities. In these so-called ion lasers, the gas in active region B is a rarified inert gas, such as argon, krypton, or xenon, at pressures in the range of about 0.3 torr to about 1.5 torr.
Whatever the type of gas utilized, however, laser emissions in active region B are stimulated by a flow of electrons F passing therethrough from a heated dispenser cathode G to an anode H located on opposite sides of bore C. Dispenser cathode G is comprised of an incandescent electrical conductor, such as tungsten, having embedded therein a source of free electrons, such as barium. Dispenser cathode G is heated by alternating current in secondary circuit loop I of a transformer T, and the impregnated electron source commences to boil off electrons. These electrons from dispenser cathode G are then induced to flow as a direct current through active region B to anode H under the influence of a current regulated direct current source J.
It is essential to the proper functioning of a laser, such as laser A, that the alignment of first and second mirrors D, E, respectively, be sustained. Otherwise, laser energy produced in active region B will not be consistently returned through active region B, and the coherent laser will cease to be produced. The heat generated in dispenser cathode G is problematic with regard to maintaining laser mirror alignment, because that heat causes dimensional changes in the laser structure surrounding dispenser cathode G. Such temperature-related dimensional changes, if circumferentially nonuniform, can cause bending in the structure intermediate first and second mirrors D, E, respectively. This will throw those mirrors out of alignment. Thus, it has been a longstanding problem in lasers using cathodes to grapple with the temperature-induced dimensional changes and the resulting mirror misalignment caused by cathode heating.
In the past, conventional wisdom has dictated that the structures surrounding such heated cathodes be made of a low expansion alloy material, such as Kovar.TM., which is itself a combination of about 42% nickel, 14% cobalt, and 44% iron. Such material is available, for example, from the Carpenter Steel Division of Carpenter Technology of Reading, Pennsylvania, and other sources in a number of forms. Nevertheless, even such special alloys do not entirely eliminate temperature-induced dimensional changes in the structures about a heated cathode.
It has, accordingly, further become the standard practice to attempt to cool by convection or induced contact with air at ambient or lower temperature conditions the structure in which such heated dispenser cathodes are located. In some instances, the immediate housing surrounding the heated dispenser cathode is provided with cooling fins and the like in order to effect the maximum possible removal of heat therefrom. It was thus the goal to maintain the cathode housing at the lowest possible temperature. This strategy to solving the problem of laser mirror alignment relied on minimizing the magnitude of temperature-induced dimensional changes. With cathode housings exposed to cooling mediums at ambient or lower temperatures, the housings themselves were maintained in a temperature range of about 130.degree. C. to about 400.degree. C.
Typically, ion lasers employing dispenser cathodes require a reserve volume, or ballast, of the gas utilized in active region B for generating laser phenomena. In the low pressure setting of the ion laser this reserve constituted a small mass which was allowed to communicate freely with active region B. Typically, in order to permit the housing of the heated cathode used with such lasers to be directly contacted by a coolant at ambient or lower temperature conditions, the ballast was housed in a reservoir which was longitudinally separated from the cathode housing. In this way, the physical presence of the volume of the ballast did not impede the cooling effect of the ambient surroundings on the cathode housing.
Nevertheless, such efforts to ensure mirror alignment in lasers using dispenser cathodes have not proved satisfactory. Mirror misalignment is a common operating problem, requiring the continual resetting of mirror orientations in lasers to ensure their functioning. The use of cooling fins on the exterior of the cathode housings in such lasers has added complexity to the devices, and the need to expose the cathode housing to a cooling fluid at ambient or lower temperature conditions has limited the freedom of design available for other aspects and the overall configuration of lasers. Until the present invention, no satisfactory approach had been achieved to the maintenance of mirror alignment in lasers utilizing a heated dispenser cathode intermediate therebetween. This problem is particularly acute in the smaller, low-cost lasers finding ever wider use in industry, research, and medicine. In shorter lasers the length of the portion devoted to housing the dispenser cathode becomes a significant fraction of the total overall length. Under such circumstances temperature-related dimensional changes in the overall device become extremely significant.